24
RT-SVX069D-EN
Start Up
Electromechanical Controls -
Sequence Of Operation
These units are offered with electromechanical controls.
Cooling without an Economizer
When the thermostat switch is set to the "Cool" position
and the zone temperature rises above the cooling setpoint,
the thermostat Y contacts close. The compressor contactor
(CC1) coil is energized provided the low pressure control
(LPC1), high pressure control (HPC1) and discharge line
thermostat (DLT 1) are closed. When the (CC1) contacts
close, compressor (CPR1) and both outdoor fan motors
(ODM1 and ODM2) start. If the first stage of cooling cannot
satisfy the cooling requirement, the thermostat closes Y2.
The compressor contactor (CC2) coil is energized provided
the low pressure control (LPC2), high pressure control
(HPC2) and discharge line thermostat (DLT 2) are closed.
When the (CC2) contacts close, compressor (CPR2) starts.
Evaporator Fan Operation (for Gas Units)
- Standard Economizer
Economizer is a field installed accessory. Refer to the
following economizer information when the accessory kit is
installed.
When the thermostat fan selection switch is set to the
"Auto" position, the Ignition Module (IGN) energizes the
indoor fan relay (F) approximately 1 second after
energizing the compressor contactor coil (CC1) in the
cooling mode. In the heating mode, the Ignition Module
(IGN) energizes the indoor fan relay (F) coil approximately
45 second after gas ignition. Closing indoor fan relay (F)
coil starts the indoor fan motor (IDM). The (IGN) de-
energizes the fan relay (F) approximately 80 seconds after
the cooling requirement has been satisfied to enhance unit
efficiency.
When the heating cycle is terminated, the indoor fan relay
(F) coil is de-energized approximately 90 seconds after the
heating requirement.
When the thermostat fan selection switch is set to the "On"
position, the (IGN) keeps the indoor fan relay coil (F)
energized for continuous fan motor operation.
Evaporator Fan Operation (for Cooling
Only Units) - Standard Economizer
When the thermostat fan selection switch is set to the
"Auto" position, the thermostat energizes the indoor fan
relay coil (F) to start the indoor fan motor (IDM).The fan
relay (F) de-energizes after the cooling requirement has
been satisfied. When the heating cycle is terminated, the
indoor fan relay (F) coil is de-energized with heater
contactors.
When the thermostat fan selection switch is set to the "On"
position, the thermostat keeps the indoor fan relay coil (F)
energized for continuous fan motor operation.
Economizer Set-Up - Standard
Economizer
Adjusting the minimum position potentiometer located on
the unit economizer module (RTEM-EM) sets the required
amount of ventilation air.
Ambient temperature is controlling the economizing cycle
by sensing the outside air dry bulb temperature.
lists the selectable dry bulb values by potentiometer
setting.
Table 7.
Potentiometer settings
Potentiometer
Setting
Dry Bulb
Enthalpy
A
27 Btu/lb (63 kJ/kg)
73°F (22.8°C)
B
25 Btu/lb (58 kJ/kg)
70°F (21.1°C)
C
(a)
23 Btu/lb (53 kJ/kg)
67°F (19.4°C)
D
22 Btu/lb (51 kJ/kg)
63°F (17.2°C)
E
19 Btu/lb (44 kJ/kg)
55°F (12.8°C)
(a)
Factory setting
Cooling with an Economizer - Standard
Economizer
The economizer is utilized to control the zone temperature
providing the outside air conditions are suitable. Outside air
is drawn into the unit through modulating dampers.
When cooling is required and economizing is possible, the
unit economizer module (RTEM-EM) opens the
economizer damper. The RTEM-EM continues to modulate
the economizer damper open/closed to keep the mixed air
temperature in the 50°F to 55°F range.
The thermostat will close the Y2 contacts to turn on
contactor (CC1) if mechanical cooling is required.
If economizing is not possible, the RTEM-EM drives the
damper to the minimum position setpoint when the indoor
fan relay (F) is energized and allows mechanical cooling
operation.
Heating Operation (Units with Electric
Heat) - Standard Economizer
When the system switch is set to the "Heat" position and
the zone temperature falls below the heating setpoint, the
thermostat closesW1 contacts the first stage electric heat
contactor (AH or AH and CH) is energized. If the first stage
of electric heat cannot satisfy the heating requirement, the
thermostat closes W2.
